Silica aerogel owns both of high light transmittance and thermal insulation properties and low refractive index and dielectric properties, which are as low as those of gas, and accordingly, is expected to be fully used for a variety of purposes including a transparent insulator as an example.
As a method for producing the silica aerogel as described above, for example, a method described in Patent Literature 1 is known. Patent Literature 1 discloses a method, in which a nonionic surfactant is dissolved into an acidic aqueous solution, a metal compound having a hydrolyzable functional group is added to a resultant solution, and a product thus obtained is solidified, followed by drying. Then, Patent Literature 1 describes that an inorganic porous body obtained by the method of Patent Literature 1 includes: through holes, which have a pore diameter of 200 nm or more, and continue with one another in a three-dimensional network shape; and pores, which are formed in inner wall surfaces of the through holes, and have a pore diameter ranging from 5 to 100 nm.
Here, alcogel for use in producing the silica aerogel has micropores formed in such a manner that a silicon compound is subjected to polycondensation. However, in general, capillary force applied to the micropores is increased as the pore diameter is smaller and a surface tension of a solvent is large, and accordingly, the alcogel becomes prone to be broken. As a method for avoiding this, there is considered a method of drying the alcogel under a supercritical condition using carbon dioxide. However, such drying under the supercritical condition is a high-pressure process, and accordingly, a large amount of capital investment is required for a special device and the like, and much more labor and time are also required.
Therefore, as a method of drying the alcogel more simply, there is examined a method of imparting, to the alcogel, framework strength exceeding the capillary force or framework flexibility that enables free deformation following the capillary force. Patent Literature 2 discloses a method including: a step of converting sol into gel after preparing the sol by adding a silicon compound such as methyltrimethoxysilane into an acidic aqueous solution containing a surfactant; and a step of drying the sol. Moreover, Patent Literature 2 describes that the gel is dried at a temperature and a pressure, which are those less than a critical point of the solvent for use in drying the gel.